Particle Image Velocimetry

Particle Image Velocimetry (PIV) is a non- optical method for determination of the velocity fields in the fluid mechanics. In a short time interval particles are photographed in the fluid. From the particle positions in the pictures, the movement direction and speed can be approximately averaged.

Principle

A fluid to be investigated are added to the smallest particles. For flow analysis in water usually microscopic air-filled glass balls are used. Also suitable are plastic particles of polyamide. In gases, the particles are produced for example, from a conventional cooking oil or a test fluid such as DEHS by compressed air. The diameter of the particles is between 0.5 and 50 microns. The density of the particles should be selected to be similar to the density of the test fluid. An expanded laser beam illuminates on a plane in a plane, the particles pulsating. During a pulse, two images are shot at a short distance. Mostly this two CCD chip of a camera can be used. The time interval of the tripping delay must be adjusted to the main flow velocity. The faster the flow, the shorter the distance the trigger delay must be chosen. The particles move in the time between the two images with the local flow velocity. The reflected light from the particles of the two pulses is represented with a lens onto the CCD sensor of a camera, and then digitally processed.

Two imaging techniques

For each light pulse own picture of the flowing particles is recorded. The determination of velocity components in the image plane can be achieved by calculating the cross-correlation function between neighboring image regions in the two images. With the methods of photogrammetry, the camera can be calibrated so that the movement of the image coordinates into spatial coordinates can be converted.

Frame method

A variant of the method uses a single image for analysis. The intensity of the light pulse is varied during the exposure of the image. Thus each particle track is characterized by its movement in the image, whose brightness decreases between beginning and end. The track can also be assigned a direction and speed information. The following 3-D evaluation is carried out exactly as in the two imaging methods.